KVM: x86: Prevent excluding the BSP on setting max_vcpu_ids

If the BSP vCPU ID was already set, ensure it doesn't get excluded when
limiting vCPU IDs via KVM_CAP_MAX_VCPU_ID.

[mks: provide commit message, code by Sean]

Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20240614202859.3597745-4-minipli@grsecurity.net


Signed-off-by: Sean Christopherson <seanjc@google.com>